Capacitive micro-machined ultrasound transducer (CMUT) are a well-known technology for the use in ultrasound imaging applications and provide a possibility for a low-cost replacement of ultrasound transducers based on piezoelectric technology.
CMUT cells comprise a cavity underneath a flexible membrane. For detecting ultrasound waves, a vibration of the flexible membrane, which move or vibrate according to the receiving ultrasound waves, can be detected by measuring a variation of the capacitance between electrodes of the flexible membrane and a substrate of the CMUT cells. Conversely, an electrical signal applied to the electrodes of the CMUT cells cause the membrane to vibrate and thereby to emit ultrasound waves.
To increase the sensitivity of the CMUT cells, a “collapse mode” has been developed, wherein a DC bias voltage is used to bring the membrane into contact with the CMUT substrate and whereby the sensitivity of the cells can be doubled. A collapsed mode operable CMUT including a contoured substrate is e.g. known from US 2011/0040189 A1.
However, the CMUT cells operated in the collapse mode are subjected to electric charging and dielectric breakdown, whereby the lifetime of the CMUT cells is significantly reduced. Improved CMUT structures have been developed which can be operated in the collapse mode with high sensitivity having a longer dielectric life time, however, it has to be assured that the dielectric components of the CMUT cells do not charge or breakdown during the life time of the ultrasound transducers.
From US 2007/0140515 A1 a transducer static discharge apparatus is known, wherein light is exposed to capacitive membrane transducer elements of an ultrasound transducer assembly in order to reduce the static charge inside the transducer elements.
From US 2005/0119575 A1 a capacitive microfabricated transducer array for 3-D imaging is known, comprising a relatively large elevation dimension and a bias control of the elevation aperture in space and time.
From US 2012/0194107 A1 a control apparatus for a capacitive electromechanical transducer is known comprising cells each including first and second electrodes facing each other via a gap, a driving/detecting unit and an external stress applying unit, wherein the driving/detecting unit causes the second electrode to vibrate and to transmit elastic waves by generating an AC electrostatic attractive force between the electrodes, or to detect a charge of capacity between the electrodes, wherein the charge is caused by the second electrode vibrating upon receipt of elastic waves.